Liquefied petroleum gas (LPG) may be used as a fuel for an internal combustion engine. LPG may be primarily comprised of propane and it has a relatively low super critical temperature of about 96° C. If LPG is elevated to temperatures greater than its critical temperature, it may be supplied to an engine in an unknown density, somewhere between a gaseous and liquid state. If LPG is supplied to the engine at temperatures less than its critical temperature, it may be supplied to the engine via fuel injectors in a liquid state. LPG in a liquid phase exits a fuel injector and flashes to a gaseous state with great rapidity. Supplying LPG in a liquid state may be desirable because liquid fuel may be supplied directly into a cylinder where it evaporates and cools the cylinder air-fuel mixture so that the engine may tolerate additional spark advance and be less prone to engine knock. However, engine compartment temperatures may reach levels higher than the critical temperature of LPG. Consequently, there may be conditions when LPG changes state to supercritical before being injected to the engine. Injecting a desired amount of fuel becomes difficult due to the unknown fluid density. The fuel's state change from liquid to supercritical fluid may result in engine air-fuel ratio errors.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for operating an engine, comprising: cooling a direct injection fuel pump with a liquid fuel, the liquid fuel not pumped via the direct injection fuel pump.
By cooling a direct injection fuel pump, or fuel pump and fuel rail, with a fuel that is not pumped by the direct injection fuel pump, it may be possible to supply fuel to the engine in known states so that the possibility of air-fuel errors may be reduced. For example, a fuel that cools the direct fuel injection pump and/or fuel rail evaporates and may be injected to the engine via a port fuel injector. On the other hand, fuel pumped through the cooled direct injection fuel pump may be injected directly into engine cylinders via direct injectors. In this way, the engine may be supplied fuel in known states so that fuel injection timing may be adjusted to provide a desired ratio of air and fuel to the engine. In one example, the gaseous fuel may be supplied to the engine when the direct injection fuel pump temperature is high enough to vaporize fuel pumped by the direct injection fuel pump. Liquid fuel may be supplied to the engine when direct injection fuel temperature is low enough to pump fuel through the direct injection fuel pump in a liquid state.
To achieve a desired amount of under hood fuel cooling, the engine system may source the first portion of it fuel via vapor. As the vapor injection pressure diminishes due to fuel cooling, the balance of the fueling may be sourced via the liquid injection system (e.g., direct fuel injection). During conditions where maximum engine power is desired, engine power may be increased by injecting most of all of the fuel supplied to the engine as a liquid into engine cylinders. For a hot under hood conditions where the fuel flow rate is low, all fuel provided to the engine may be injected in a gaseous state. Further, at cold ambient temperatures, all the fuel injected may be injected as liquid. Additionally, anytime vapor injection is occurring, so is direct injection pump and/or direct injection rail evaporative cooling. At high fuel flow rates where the injection method is primarily liquid, direct injection pump and direct injection fuel rail cooling occurs via replacing the hot fuel with cooler fuel from the fuel tank.
The present description may provide several advantages. In particular, the approach may reduce engine air-fuel ratio errors by allowing fuel to be injected in a known state. Further, the approach may remove a large amount of heat from a direct injection fuel pump via leveraging evaporative cooling. Further still, the approach may also improve the way boost is provided to an engine.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.